FIG. 5 shows a cross section of a prior art single wavelength oscillation semiconductor laser as disclosed in "Optics," Vo.15, No. 2, pp. 115 to 121. In FIG. 5, the reference numeral 1 designates an n type InP substrate. A diffraction grating 20 having a phase shift region 21 at the central portion is disposed at the surface of the substrate 1. A guide layer 3 comprising InGaAsP is disposed on the substrate 1 having the diffraction grating 20. An InGaAsP active layer 4 is produced on the guide layer 3 and a p type InP substrate 5 is produced on the active layer 4. Electrode metals 7 are provided at the upper and the lower surfaces of the substrates 5 and 1, respectively, and non-reflection coating films 8 are provided at the both side surfaces of the laser.
The device will be operated as follows.
In this semiconductor laser, electrons in the n type InP substrate 1 and holes in the p type InP substrate 5 are both injected into the InGaAsP active layer 4 and produce light emitting recombinations therein. In the distributed feedback (DFB) laser having a diffraction grating at the active region, light generated by the recombinations is reflected by the diffraction grating 20 and reverberates inside the element, thereby achieving laser oscillation. Then, since a phase shift region 21 is provided at the central portion of the diffraction grating 20, the oscillation wavelength becomes a single wavelength called the Bragg wavelength which is determined by the pitch of the diffraction grating 20.
The proportion of the light generated at the active region that is reflected by the diffraction lattice 20 increases as the coupling coefficient .kappa. between the light and diffraction grating 20 increases. When the coupling coefficient .kappa. is high, the proportion of the light which is confined inside the element by diffraction grating 20 is large, and the laser oscillates at a low threshold current. At the high output operation, however, the light density at the central portion of the element begins too high, and the laser becomes to oscillate at multiple wavelengths, causing localized hole burning. To the contrary, when the coupling coefficient .kappa. is low, the single wavelength property is weakened and the threshold current is increased. Accordingly, it is undesirable to achieve high power output by lowering the coupling coefficient.
Thus, in the prior art single wavelength oscillation semiconductor laser, the light density at the central portion of the laser becomes too high at high power output operation, thereby causing oscillation at multiple wavelengths.